Agree, it's hard to help you here without screenshots. Maybe the font configurations of GTK are helpful too. (GTK+ 2.x: ~/.gtkrc-2.0, GTK+ 3.x: $XDG_CONFIG_HOME/gtk-3.0/settings.ini. In these files you can find keys like gtk-icon-theme-name, gtk-theme-name and gtk-font-name.
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jens-naFeb 19 '13 at 8:38

I'm not near the CentOS machine for a few days, so no screenshots. That said, if I had to guess, I'd say antialiasing wasn't happening. I've played with settings in ~/.gtkrc-2.0. (No gtk3 on CentOS.) Those do affect the fonts in the apps GUI, but they do not affect the fonts in the actual display of items from the feeds.
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joncFeb 19 '13 at 18:06

Oops, sorry. RSSOwl is an RSS feedreader. Hence, the mention of "items from the feeds."
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joncFeb 19 '13 at 18:32